Resolution of micro-displays has increased rapidly as more advanced methods become available to pattern subpixels to the sizes less than 3.5 microns in the shortest dimension. Soon, subpixel dimensions of 2 microns or even less are anticipated. Organic light emitting diode (OLED) displays can be fabricated by several methods, including inkjet printing and vacuum deposition through shadow mask. The former method is widely used in fabrication of large format displays suitable for TV screens. The latter method is well suited for small format high resolution micro-displays. Recent success in direct patterning of OLED micro-displays with red, green and blue subpixels formed side-by-side can be credited to the development of the advanced high-resolution shadow mask with the openings on the scale of few microns. The implementation of such a high-resolution shadow mask faces several challenges. Among them is the alignment of the mask to the substrate containing backplane circuitry and pre-patterned active electrode, i.e., anode or cathode.
Alignment of a shadow mask against a substrate is typically achieved by aligning a special alignment mark, i.e., a fiducial, having a particular pattern pre-formed on a surface of the substrate against a fiducial of another particular pattern pre-formed on the shadow mask. There may be several fiducials in different locations on both the substrate and the mask for alignment observation purposes. The alignment occurs via optical inspection and analysis of the image formed by overlapping two fiducial patterns while the substrate and the mask are in contact. In such image analysis, a mathematical algorithm is used to verify if the two fiducial patterns are perfectly aligned as designed or if there are minute shifts between the two fiducials that need to be executed. Also, the deposition of organic material through the shadow mask is preferable in the configuration where both the substrate and the shadow mask are in full contact. This helps to avoid the deposited material reaching points beyond the area outlined by the shadow mask opening, i.e., to avoid a feathering effect, which is critical for the dimensional characteristics of the micro-display subpixels. Thus, during the organic material deposition, the substrate and the shadow mask must be in full contact for continuous alignment inspection and reduction of feathering.
During the organic material deposition, which is a thermal process, the kinetic energy of molecules arriving at the surface of the substrate may affect thermal conditions of the system and result in slight drift of the substrate against the shadow mask, i.e., the substrate-mask configuration can drift away from the perfect alignment, and the need for the re-alignment may arise as a result. The criteria for the alignment accuracy, or in other words the drift tolerance can be set prior to deposition. For example, for subpixel dimensions of 2-3 microns, the threshold, beyond which the alignment can be deemed inadequate, may be set to 100-200 nanometers or even less. While from the continuous alignment inspection and deposition process standpoint it is beneficial to have a full substrate-to-mask contact, the process of alignment itself will require movement of the shadow mask against the substrate, and such movement cannot be carried out while both are in full contact, and this presents a problem.